A Reassessment of the Nomenclature of Polychlorinated Biphenyl (PCB) Metabolites

Article excerpt

Polychlorinated biphenyls (PCBs) are a widespread class of persistent organic chemicals that accumulate in the environment and humans and are associated with a broad spectrum of health effects. PCB biotransformation has been shown to lead to two classes of PCB metabolites that are present as contaminant residues in the tissues of selected biota: hydroxylated (HO) and methyl sulfone (MeS[O.sub.2]) PCBs. Although these two types of metabolites are related structures, different rules for abbreviation of both classes have emerged. It is important that a standardized nomenclature for the notation of PCB metabolites be universally agreed upon. We suggest that the full chemical name of the PCB metabolite and a shorthand notation should be adopted using the International Union of Pure and Applied Chemistry's chemical name/original Ballschmiter and Zell number of the parent congener, followed by the assignment of the phenyl ring position number of the MeS[O.sub.2]- or HO-substituent. This nomenclature provides a clear, unequivocal set of rules in naming and abbreviating the PCB metabolite structure. Furthermore, this unified PCB metabolite nomeclature approach can be extended to the naming and abbreviation of potential metabolites of structurally analogous contaminants such as HO-polybrominated biphenyls and HO-polybrominated diphenyl ethers. Key words: hydroxylated metabolites, methyl sulfone metabolites, nomenclature, polychlorinated biphenyls. doi:10.1289/ehp.6409

Nomenclature of Polychlorinated Biphenyls

Polychlorinated biphenyls (PCBs) are a class of chemical compounds in which 1-10 chlorine atoms are attached to a biphenyl backbone. Theoretically, 209 discrete congeners are possible (Ballschmiter and Zell 1980). However, PCB technical mixtures are composed of a smaller suite of congeners (Frame et al. 1996), and only about 80-100 PCB congeners are of actual environmental relevance (de Voogt et al. 1990).

The full chemical notation for these 209 possible PCB congeners is inconvenient, and therefore various shorthand notations have been developed and adopted (Erickson 1997, 2001). Ballschmiter and Zell (1980) originally introduced a system (BZ) in which congeners were arranged in the ascending numerical order based on the number of chlorine atoms and their substitution pattern on the biphenyl base structure. Minor theoretical discrepancies in the BZ naming system were later corrected (Ballschmiter et al. 1992; Guitart et al. 1993; Schulte and Malisch 1983). The BZ system of PCB shorthand notation was subsequently recognized by the International Union of Pure and Applied Chemistry (IUPAC) (U.S. Environmental Protection Agency 2003) and is the generally accepted notation used by scientists who perform congener-specific PCB research.

Metabolism of PCBs

PCBs that accumulate in biota are subject to elimination processes that are facilitated by processes including enzyme-mediated degradation. The mechanism and kinetics of PCB biotransformation depend on a number of factors, including the metabolic capacity of the organism and the PCB congener structure. PCB biotransformation has been shown to lead to two classes of PCB metabolites that are present as contaminant residues in the tissues of biota that have been studied: hydroxylated (HO) and methyl sulfone (C[H.sub.3]S[O.sub.2]) PCBs (Letcher et al. 2000a). C[H.sub.3]S[O.sub.2-]PCBs are generally referred to as MeS[O.sub.2]-PCBs. The numbers of animals and populations where tissue residues of these PCB metabolites have been characterized remains small, but HO-PCBs and MeS[O.sub.2]-PCBs are emerging as common contaminant phenomena in wildlife and humans and are of increasing importance in risk assessments of exposure to PCBs (Bennett et al. 2002; Campbell et al. 2003; Chu et al. 2002, 2003; Guvenius et al. 2002; Hoekstra et al. 2003; Hovander et al. 2002; Letcher et al. 2000a, 2000b; Li et al. 2003; Sandala et al., in press; Sandau et al. …